Machine-washed dishes typically have more strict cleaning requirements than hand-washed dishes. For instance, after automatic dishwashing, dishes should not only be completely free of food residue, but should also be free of any visible water hardness or other mineral salt deposits that may have originated from dried water drops when there is a lack of wetting agents.
Modern automatic dishwashing agents satisfy these requirements by incorporating washing, conditioning, water softening, and rinsing active ingredients, sometimes known to the consumer as “2-in-1” or “3-in-1” dishwashing agents. Automatic dishwashing agents intended for residential use contain builders as an essential component for successful washing and rinsing. On the one hand, these builders increase the alkalinity of the washing liquor, which aids in the saponification and emulsification of fats and oils. On the other hand, builders reduce the water hardness of the washing liquor by complexing the calcium ions present in the aqueous liquor. Alkali metal phosphates have proved to be particularly effective builders, and for that reason they form the main ingredient in a majority of automatic dishwashing agents.
Although phosphates are highly desirable in automatic dishwashing agents for the performance benefit, their use is problematic from an environmental standpoint. A significant portion of the phosphate used in dishwashing passes with the domestic wastewater into standing bodies of water such as lakes and reservoirs to contribute to eutrophication or over-fertilization. As a consequence of this phenomenon, the use of pentasodium triphosphate in laundry detergents has been extensively regulated in a number of countries including the USA, Canada, Italy, Sweden, Norway, and has been entirely prohibited in Switzerland. In Germany, since 1984, the content of this builder permitted in washing agents has been limited to 20%.
In addition to nitrilotriacetic acid, sodium aluminum silicates (zeolites) are primarily used as phosphate replacements or substitutes in textile washing agents. However, for various reasons, these substances are not suitable for use in automatic dishwashing agents. As a consequence, various alternatives to alkali metal phosphates, most particularly citrates, have been discussed in the literature for use in automatic dishwashing agents.
European patents EP 662 117 B1 (Henkel KGaA) and EP 692 020 B1 (Henkel KGaA), for example, describe phosphate-free automatic dishwashing agents which, in addition to a citrate, furthermore contain carbonates, bleaching agents and enzymes.
As another alternative to alkali metal phosphates, and preferably used in combination with citrates, is methylglycinediacetic acid (MGDA). For example, European patent EP 906 407 B1 (Reckitt Benckiser) and European patent application EP 1 113 070 A2 (Reckitt Benckiser) describe MGDA-containing automatic dishwashing agents.
Despite the efforts made to date, manufacturers of automatic dishwashing agents have failed to provide phosphate-free automatic dishwashing agents that are comparable to, or that surpass, phosphate-containing cleaning agents with regard to their washing and rinsing performance and their ability to inhibit film deposition. Such equality of performance is a prerequisite for the successful market introduction of phosphate-free cleaning agents, since the majority of end consumers, despite the widespread public discussion of environmental issues, will always decide against an environmentally advantageous product if this product is not in line with the market standard in terms of price and/or performance.
In light of this background, the need still exists for phosphate-free automatic dishwashing agents that have at least comparable washing and rinsing performance to conventional phosphate-containing cleaning agents. Even more desirable and needed are new phosphate-free automatic dishwashing that can surpass conventional phosphate-containing dishwashing compositions in washing performance, rinsing performance, and inhibition of film deposition.